Wireless transmit/receive units (WTRUs) in a UTRAN may be in either an idle mode or a connected mode. Based on WTRU mobility and activity while in the connected mode, the universal terrestrial radio access network (UTRAN) may direct the WTRU to transition between a number of radio resource control (RRC) sub-states: Cell_PCH, URA_PCH, Cell_FACH, and Cell_DCH states. User plane communication between the WTRU and the UTRAN is only possible while in Cell_FACH and Cell_DCH states. The Cell_DCH state is characterized by dedicated channels in both the uplink and the downlink. On the WTRU side, this corresponds to continuous transmission and reception and can be demanding on user power requirements. The Cell_FACH state does not use dedicated channels and thus allows better power consumption, at the expense of a lower uplink and downlink throughput.
The Cell_FACH state is well-suited for signalling traffic, (e.g., transmission of a CELL UPDATE message), and for applications requiring very low uplink throughput. Uplink communication is achieved through a random access channel (RACH) that is mapped to a physical random access channel (PRACH). The RACH is a contention-based channel with a power ramp-up procedure to acquire the channel and to adjust transmit power. Downlink communication is through a forward access channel (FACH) that is mapped to a secondary common control physical channel (S-CCPCH). System information including the setup details for the uplink (i.e., RACH) and downlink (i.e., FACH) channels to be used in Cell_FACH is read from a broadcast channel (BCH).
In the Cell_FACH state, mobility is handled autonomously by the WTRU. The concept of soft handover does not currently (as of Release 6 of the third generation partnership project (3GPP) standard) exist within Cell_FACH. A WTRU independently takes measurements, and determines which cell to camp on.
The possibility of using high speed downlink packet access (HSDPA) in the Cell_FACH state, (i.e., enhanced Cell_FACH state), has been studied by the standardization bodies. HSDPA is a feature that was introduced in Release 5 of the 3GPP specifications to operate in the Cell_DCH state. HSDPA tries to make better use of the downlink shared capacity by using three key concepts: adaptive modulation and coding (AMC), retransmissions using a hybrid automatic repeat request (HARQ) scheme, and Node-B scheduling—all operating at a very fast rate.
Every WTRU having an HSDPA connection is assigned an HS-DSCH radio network temporary identifier (H-RNTI). The H-RNTI is unique within a cell and assigned by the serving radio network controller (SRNC). A WTRU is attached to a single serving cell, (i.e., Node B). The WTRU has to be told about the physical channel resources to use (high speed physical downlink shared channel (HS-PDSCH) information), as well as how to set up the HARQ processes and the HARQ memory.
As a result of mobility, the WTRU may change from one serving cell (source Node B) to another (target Node B). The UTRAN controls the timing of this change. The SRNC must stop sending data to the source Node B and begin sending data to the target Node B with the new configuration. At the same time, the RNC must send a control message (RRC message) to reset the HS-DSCH medium access control (MAC-hs) entity at the WTRU.
The MAC-hs reset would entail flushing soft buffer for all configured HARQ processes; stopping all active re-ordering release timers (T1) and setting all timers (T1) to their initial value; starting transmission sequence number (TSN) with value 0 for the next transmission on every configured HARQ process; initialising the variables RcvWindow_UpperEdge and next_expected_TSN to their initial values; disassembling all MAC-hs protocol data units (PDUs) in the re-ordering buffer and delivering all dedicated channel medium access control (MAC-d) PDUs to the MAC-d entity; flushing the re-ordering buffer; and in some cases, indicating to all acknowledged mode (AM) radio link control (RLC) entities mapped on a high speed downlink shared channel (HS-DSCH) to generate a status report.
When trying to transmit HSDPA while in the enhanced Cell_FACH state, there are many problems that need to be addressed. HSDPA is currently standardized to operate only in the Cell_DCH state. The WTRU makes use of variable HS_DSCH_RECEPTION to verify whether or not HSDPA reception is allowed. In the enhanced Cell_FACH state, the WTRU receives common channel set up information in the broadcast system information. However, no HSDPA configuration information is carried in the broadcast information.
The enhanced Cell_FACH state will use downlink signalling radio bearers on common logical channels (common control channel (CCCH) and common traffic channel (CTCH)). Typical RRC messages carried on these radio bearers include RRC CONNECTION SETUP message and CELL UPDATE CONFIRM message. The former message poses a problem as the HSDPA configuration details are included inside this message. For Cell_DCH, WTRUs wait until reading the configuration details before allowing HSDPA communication. This is not possible for enhanced Cell_FACH, as the message must be received using HSDPA communication. Conventional 3GPP Release 6 specifications do not provide support for operation of HS-DSCH in Cell_FACH.
When a WTRU is in an enhanced Cell_FACH state, the WTRU will execute the cell update procedure for a number of reasons, (e.g., cell reselection, radio link failure, radio link control (RLC) unrecoverable error, etc.). With respect to the cell update procedure, many difficulties may arise. For example, the WTRU may be asked to transition to enhanced Cell_FACH, but it requires a way to retrieve HSDPA configuration information. The cell reselection procedures are controlled by the WTRU. As a result, the UTRAN is not able to execute a timely and synchronized MAC-hs reset. In fact, after a cell reselection, the source Node B would continue to send information to the WTRU, even though the latter has stopped listening. The UTRAN would only be aware of the change after receiving a CELL UPDATE message. An additional problem may occur if the WTRU needs to send an RLC status report as a result of the MAC-hs reset. After a cell reselection, the UTRAN is notified with a CELL UPDATE message. The UTRAN responds with a CELL UPDATE CONFIRM message using a dedicated control channel (DCCH). This message has to be sent to a dedicated WTRU but the WTRU has not yet been assigned a dedicated H-RNTI (the information would typically be contained within the message itself).
When a WTRU is in an enhanced Cell_FACH state, it is generally agreed that the MAC-c/sh functionality can be reduced. In particular, the WTRU identity (ID) is already carried in the MAC-hs header and as a result it need not be repeated in the MAC header.